1. Field of the Invention
The invention relates to mixing apparatus for producing blends of liquids, solids suspensions, and gas dispersions, at low and controllable shear.
2. Prior Art
Various different types of impellers are used for different mixing applications. Among other purposes, mixing may involve any of various procedures in which agitation, flow or other movement is produced in a material, and an impeller affects the movement. Typically the impeller is moved relative to a vessel containing the liquids or solids to be mixed, but that is just one possible configuration.
Mixing often is used in an effort to achieve uniformity of blend or solids suspension, crystallization and dispersion of immiscible fluids or gases, etc., and this disclosure, without limitation, generally refers to examples in which such mixing is involved.
The degree of mixing obtained over a given mixing duration, and/or the duration of mixing needed to achieve a given degree of mixing, depends in part upon the rate at which mechanical energy is transferred from the impeller to the fluid. Transfer of energy can be a complex process, and takes place throughout the entire mixing domain. The intensity of energy dissipated locally within a vessel varies with location. A high proportion of energy dissipation occurs in the impeller zone, where a total of 20% to 25% of the energy supplied in generating relative movement of the impeller is dissipated in the impeller swapped volume. The swapped volume is the volume where the moving impeller blades encounter product on a leading side of the blades, displace the product, and where other product fills in on the trailing side of the blades.
Products containing delicate particle agglomerates, polymers such as latex, living organisms and other such products, can be damaged by high levels of shear. High shear results from vigorous mixing characterized by a high rate of dissipation of energy into the product. Inasmuch as the transfer of energy is most concentrated in the impeller zone, it would be advantageous when mixing such products to reduce local shear at the impeller to a minimum. Reducing shear reduces the transfer and dissipation of energy into the local impeller zone.
One way to decrease local energy dissipation at an impeller is to increase the projected height or axial extension of the impeller blades. This increases the volume of product that is encountered by the impeller blades and “swapped” as the impeller passes during each impeller rotation. Assuming equal process power is applied to a rotating impeller shaft (i.e., equal torque and rate of rotation), changing to an axially longer impeller will spread the same energy over a larger swapped volume. That reduces local dissipation because the dissipation is less concentrated.
Increasing the swapped volume by increasing the radial dimension of the blades also increases the impeller volume and spreads the energy over a larger volume. However, the radially outer part of an impeller has a higher linear speed if the radius is made longer, which is a factor that increases local shear at the radially outer parts of the impeller blade.
Although dissipated energy from an axially extended larger impeller is spread over a larger swapped volume, the apparatus can still be effective for mixing. The larger impeller affects a larger local impeller zone, and the overall batch may still be well mixed.
The idea of enlarging the size of an impeller blade in this way so as to encounter or swap a larger volume and achieve mixing at less concentrated localized shear can be found, for example, in U.S. Pat. No. 6,508,583 Shankwitz, at al., U.S. Pat. No. 5,399,014 Takata, at al., and U.S. Pat. No. 6,331,071 Akamine, at al.
The foregoing listed patents provide mixing configurations wherein the impeller is characterized by substantially vertical impeller blades, i.e., flat or planar shapes extending radially from and axially along a rotating vertical impeller shaft. This structure produces predominantly a rotational and/or radial flow of product in the impeller zone, and is relatively inefficient for mixing. The movement of the product due to rotation of the impeller is relatively limited to rotating rather than otherwise moving the impeller volume, namely that portion of the product that is in the path of the blades of the rotating impeller. Radial impellers are prone to stratification of the batch in the mixing vessel.
It would be advantageous to provide a configuration in which a low shear impeller is more efficient with respect to its mixing efficiency, without sacrificing the benefits of spreading the dissipation and shear over a large impeller swapping volume.